Mutation of the APC tumor suppressor gene is believed to be a rate-limiting and necessary event in the development of most colorectal tumors, both inherited and sporadic. The APC gene product is a component of the Wnt signaling pathway;its ability to interact with and down-regulate beta-catenin is controlled by GSK3IB phosphorylation and affects the expression of genes regulated by transcription factors of the TCF/LEF family (Clevers and van de Wetering, 1997). Therefore, mutation of APC indirectly alters the transcription profile of cells through changes in the expression of genes such as c-myc and cyclin D1 that promote cell growth (He et al., 1998;Tetsu and McCormack, 1999), survivin that inhibits apoptosis (Zhang et al., 2001), and matrilysin that mediates differentiation (Crawford et al., 1999). The role of APC in non-transcriptional mechanisms of growth control and differentiation is less emphasized, but includes effects on cell adhesion, microtubules and chromosome dynamics (Nathke et al., 1996;Kaplan et al., 2001;Mogensen era/., 2002;Bienz and Hamada, 2004). Further functional characterization of APC and its gene product in normal and malignant cells is important for a complete understanding of how its disruption is associated with tumor formation. We have shown that introduction of the APC gene into colon carcinoma cells induces apoptosis and prevents entry into S-phase of the cell cycle through beta-catenin-dependent and -independent mechanisms (Heinen et al., 2002;Carson et al., 2004;Qian et al, 2005). Transcriptionally silent in vitro systems show that full-length APC and defined protein segments accelerate apoptosis and inhibit DNA replication (Steigerwald et al., 2005;Qian et al., 2005;Sarnaik et al., 2005). In addition, we have shown that APC phosphorylation by the cyclin-dependent kinase p34 at M-phase of the cell cycle mediates protein interactions that in turn affect a mitotic checkpoint (Trzepacz et al., 1997;Carson et al., 2005). We will now test the hypothesis that APC contributes to cell cycle control and cell death independently of beta-catenin-induced transcription by determining the post-translational modifications of APC and its dynamic protein-protein interactions. Our immediate goal is to determine the mechanisms by which APC regulates the cell cycle and apoptosis. Our long-term goals are to elucidate the mechanisms and pathways through which APC functions and, by doing so, to contribute to the design of better therapeutic strategies in oncology.